bandgapreference

A bandgap reference is an electronic circuit that generates a stable DC reference voltage by combining temperature-dependent quantities in a way that cancels their temperature variations. The best known goal is a voltage near the silicon bandgap energy, about 1.2 volts at room temperature, with minimal dependence on supply voltage and ambient temperature. Bandgap references are widely used in integrated circuits as core voltage references for analog and mixed-signal blocks.

Principle: The circuit typically uses two temperature-dependent elements: a complementary-to-absolute-temperature (CTAT) term, such as a transistor’s base-emitter voltage, which decreases with rising temperature, and a proportional-to-absolute-temperature (PTAT) term, produced from a current that causes a voltage difference that increases with temperature. By choosing device geometries and resistor values, the PTAT term can be weighted to cancel the slope of the CTAT term, yielding a near-constant output. The output voltage is commonly formed as the sum of a Vbe and a scaled ΔVbe, resulting in a stable reference around 1.2 to 1.25 volts at room temperature.

Implementation and variants: The classic approach, known as the Brokaw bandgap, uses transistors with different emitter areas to generate ΔVbe and then combines it with Vbe. Variants, such as Widlar designs, optimize for startup behavior, power consumption, or noise. In modern integrated circuits, CMOS bandgap references implement the same principle with MOS transistors and carefully matched resistors and feedback networks. There are purely CMOS versions and hybrids, all aiming to minimize temperature drift and improve line and load regulation.

Applications and considerations: Bandgap references serve as stable cores for ADCs, DACs, voltage regulators, and microcontroller primitives. Design challenges include residual temperature curvature, process variations, noise, and power-supply sensitivity. Techniques like curvature compensation, trimming, and careful layout improve accuracy and stability across operating conditions.